Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

418
FXUS63 KLOT 121758
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1158 AM CST Fri Dec 12 2025

.KEY MESSAGES...

- Accumulating snow is expected for much of the area on
  Saturday, with several inches of accumulation possible south
  of Interstate 80.

- Very cold temperatures and bitterly cold wind chills are
  expected this weekend with wind chills below zero.

- There is a growing signal for a pattern change toward milder
  (and less snowy) conditions next week.

&&

.DISCUSSION...
Issued at 253 AM CST Fri Dec 12 2025

Through Saturday Evening:

Accumulating snow is quickly departing to the ESE early this
morning for areas south of the I-80 corridor. Meanwhile, a
pocket of light lake effect snow showers developing over far
southern Lake Michigan should remain offshore as low-level flow
quickly turns west this morning. Some clearing across the
northern CWA can also be expected this morning, but thickening
mid and upper-level clouds will filter over the area through the
afternoon.

A ragged mid-level wave with weak ascent within a ribbon of
moisture will cross southern Wisconsin this afternoon and clip
northeast Illinois this evening. With marginal forcing and some
low-level dry air in place, precip should be limited to only
some sporadic flurries.

The active upper jet on the southwest extent of a longwave
trough over eastern North America will bring yet another quick-
moving system across the mid to upper-Mississippi River Valley
on Saturday. 700-800 hPa frontogenesis below the right entrance
of a 140 knot upper-jet streak will provide ample forcing within
an axis of Pacific-based moisture. Accumulating snow is
expected for much of the area, with several inches of
accumulation possible for areas south of I-80 and especially
toward central Illinois. While thermo profiles depict a deep
layer within the DGZ, more than half of that depth resides
solidly below the layer of maximum ascent. The entire thermo
profile will also be shifting colder with time, so the residence
time of maximum ascent within the deeper DGZ will last only a
few hours at any given location. However, where the upper jet
forcing and frontogenesis best align will likely result in very
high SLR values of 20:1.

Latest indications are that the axis of highest snowfall of 3-5
inches will settle just southwest of the CWA but clip areas
south of a Pontiac to Benton County line. Modest low-level dry
air advection combined with decreasing forcing with northward
extent will lead to a somewhat sharp snowfall gradient over the
CWA. It is feasible that areas north of I-88 remain dry, with
the main snowfall gradient along or near the Kankakee River
Valley. However, ongoing CAA with temps holding steady in the
upper single digits to lower teens mean lower amounts will still
result in very slippery travel conditions. In coordination with
neighboring offices along with remaining uncertainty as to
whether the highest snowfall axis affects the southern CWA, did
not issue a Winter Weather Advisory at this time. However, if
guidance remains steady over the next couple cycles, an advisory
will likely be needed for at least the southern tier of
counties and possibly as far north as I-80.

Synoptic-scale snow will quickly exist to the southeast shortly
after sunset on Saturday as strong CAA continues. Though a 5kft
inversion will limit LES potential with the WNW wind belts of
northern Porter County Saturday night, the entire cloud depth
residing in the DGZ will be quite efficient in snowfall
production. Would not be surprised to see up to a few inches of
high SLR snow during the nighttime hours.

Kluber


Late Saturday Night through Thursday:

In the wake of Saturday`s system, an arctic airmass will
descend on the region as a mid 1040s mb high builds across
Minnesota and Iowa. While it`s always difficult to trust model
guidance`s handling of low-level moisture and cloud cover in
these arctic airmasses, a significant push of dry air is
advertised with the incoming high, suggesting things should
pretty readily clear out Saturday night away from lingering lake
effect precipitation across parts of NW Indiana. With the core
of the surface high positioned solidly to our west, we`ll
maintain persistent northwesterly breezes (10-15 mph wind with
some gusts around 20 mph) into Sunday morning. There is now
strong agreement on wind chills dropping solidly into the 15 to
25 F below zero range overnight Saturday into Sunday morning,
perhaps briefly a bit colder in spots. As a result, we`re likely
headed towards Cold Weather Advisories for all of the forecast
area into Sunday morning.

It looks like we`ll still maintain a little wind through the
day on Sunday as temperatures struggle back through the positive
single digits. This will likely mean wind chills will remain
below zero through the day. The core of the surface high is now
forecast to drift just south of the region early Sunday evening.
While this should allow temperatures to plummet initially,
southwesterly winds may gradually return overnight, possibly
resulting in slowly rising temperatures into Monday morning. The
net result of this will likely be wind chills holding more-or-
less steady in the 10 to 20 below range.

Strengthening warm advection on Monday will help send temperatures
back into the teens and 20s. Thankfully, not seeing any signs of
this intensifying WAA and attendant ascent driving any meaningful
mid-level saturation, with precip-free conditions to start the week.

Medium range guidance, to varying degrees, depicts a low-
amplitude disturbance in the general region Tuesday night into
Wednesday. Intensifying southwesterly flow may help drive an
increase in low-level moisture, with some guidance (GFS and CMC
in particular) looking like they want to squeeze out some
drizzle at times. At this range, this signal remains a bit
nebulous, but air and dewpoint temperatures look to be warming
to near/above freezing.

An abrupt transition to quasi-zonal flow is advertised towards the
end of next week/next weekend with a strong signal that the deep
arctic airmass will--at least briefly--get shoved north of the
region.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1145 AM CST Fri Dec 12 2025

Key Aviation Messages for the current TAF period:

- MVFR cigs expected later this afternoon

- Northwest wind gusts increase to up to 25 knots with the
  passage of a cold front this evening.

- Less than 30 percent chance for flurries/light snow this with
  the frontal passage, but no accumulations are expected.

- Better chances for snow impacting area terminals tomorrow
  reducing vis down to IFR levels and some accumulating
  snowfall.

Conditions are fairly quiet at present, with VFR conditions and
light west-southwest winds. An arctic cold front is presently
moving through western Wisconsin and moving east. As that front
approaches northern Illinois, cloud cover will increase and
lower cigs down to MVFR levels. This front does have snow
showers accompanying it, but any impactful showers that are
reducing vis in Wisconsin are expected to remain farther north
away from IL terminals. That being said, cannot rule out some
flurries or light snow occurring between 00Z and 06Z tonight.
However, no accumulations or impacts are expected, so opted to
keep it out of the TAF. Lastly, winds will slowly become more
northwesterly this afternoon. As the front moves through, wind
gusts will increase to around 25 knots through the overnight
before diminishing around day break.

The next winter system is expected to move across Illinois on
Saturday morning. Moderate confidence on timing, first
approaching KRFD just before 15Z, before moving east toward the
Chicago terminals. While accumulating snow is expected, there is
fairly high confidence that the heaviest accumulations are
expected to the south in Central Illinois. Up to an inch is
possible near KRFD, with 1 to maybe 2 inches possible at the
Chicago terminals (with better chances for higher accumulations
to the south). Rates could approach 0.25"/hour and reduce
visibilities down to IFR levels. The TAF converted PROB30s to
TEMPOs given the increasing confidence to more pessimistic
impacts.

It is possible that flurries/light snow linger into the mid to
late afternoon as the system moves east. However, conditions
should gradually improve toward 00Z back to VFR.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 9 PM this evening to noon CST Sunday
     for Winthrop Harbor IL to Gary IN.

     Small Craft Advisory from 9 PM this evening to 9 PM CST Sunday
     for Gary to Burns Harbor IN-Burns Harbor to Michigan City
     IN.

&&

$$

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